Vacuum deposition is a common method for coating metals such as aluminum, copper, zinc, and tin onto various substrates of metal, glass, and plastic. The metal is typically vaporized by means of electric resistance heating in a metallic or ceramic container or vessel generally referred to in the art as a “boat” or a metallization boat. The boat is connected to a source of electrical power within an evacuated chamber and heated to a controlled operating temperature sufficient to cause a metal charge placed in contact with the boat to vaporize.
In the vacuum metallization process, the metal melt in a metallization boat is heated to a very high temperature, in many instances to a temperature higher than typically seen in casting operations, of up to 1200° C. and above. This means that the metal melt, i.e., aluminum melt, will be much more aggressive and behave as a corrosive acid, lowering the life of the metallization boats.
Besides the metallization conditions, a factor in the life of the boat is the wettability of the boat surface. If the surface of the boat is not substantially wettable to molten metal, higher boat temperatures will be required to evaporate a unit amount of aluminum due to smaller evaporation surface area. Higher operating temperature will consequently lead to a higher corrosion rate thus reducing the useful life of the boat.
Besides the wettability of the boat surface, another factor that is important to longer/useful boat life is the wettability of the slag build-up in the cavity of the boat. After a few hours of metallizing operations, a slag begins to build up around the edge of the metal puddle in the boat. The slag is the by-product of the reaction of the molten metal with the refractory boat. The slag is typically non-wetting to the metallizing metal and inhibits the spreading of the metal puddle, thus increases the operating temperature and further reduces the useful life of the boat. Improved wetting of the boat and/or slag surface is believed to be an important factor in increasing the average life of metallization boats.
There is yet another property/factor that is important to longer and useful boat life; it is the resistance to thermal degradation of the boat. If the boat operating temperature exceeds a critical value, e.g., in excess of 1600° C., the boat will thermally decompose, allowing the molten metal to penetrate into the boat thus decreasing the useful life of the boat. This phenomenon is typical when the wire is not fed uniformly onto the boat surface, causing the development of localized hot spots within the boat.
The average life of metallization boats in the prior art ranges is about ten (10) hours, providing from about 600,000 metallized feet of substrate to about 1,000,000 feet of substrate, depending on the metallizing conditions. There is a continuous interest in increasing the useful life of metallization boats with various refractory boat compositions. U.S. Pat. No. 4,528,120 discloses boats comprising 10–60 wt. % boron nitride (“BN”), 0–60% of at least one of an aluminum nitride and a silicon nitride, and 30–70 wt. % of one of the group of the borides of titanium, zirconium, aluminum, and chromium and the carbides of silicon, titanium and chromium. JP Publication Number 06-298566 discloses a composition comprising 0.3 to 10 wt. % of an alkaline earth metal oxide, rare earth oxide, or alkaline-earth-metal borate, 15–50 wt. % BN, and the remainder aluminum nitride and titanium diboride in a ratio of 1:1.5 to 1:4.
In U.S. Pat. No. 5,604,164 to Montgomery, it is disclosed that “the addition of an elemental metal such as molybdenum and an oxide additive such as Y2O3 to a ceramic composite composition of titanium diboride and boron nitride, has been found to substantially improve the properties of the boat, especially its wetting characteristics, and a reduction in the rate of corrosion of the boat cavity by aluminum.” Montgomery discloses a refractory boat consisting essentially of titanium diboride; boron nitride; at least 3 wt. % of a metal selected from Mo, W, Ta, and Nb; and at least 0.7 wt. % of an oxide selected from CaO and Y2O3.
There is still a need to increase useful life of metallization boats. Applicants have surprisingly found that a refractory boat composition with improved properties, including improved resistance to thermal degradation, wettability, and corrosion resistance provides longer useful life. The surprising improved composition for refractory boats in one embodiment exhibit properties of: a) less corrosion of the boat surface; b) a higher degree of thermal degradation resistance, c) more wettable boat surface; and d) more wettable slag, without the requirement of doping the composition with at least 3 wt. % of the metal element Mo, Ta, W, and/or Nb.